The present invention relates to an optical harmonic generator for converting incident light (hereinafter referred to as fundamental wave) to light with a wavelength which is an integral measure of the wavelength of the incident light (hereinafter referred to as harmonic) by means of a nonlinear optical effect, wherein phase matching between the fundamental wave and the generated harmonic, which is necessary to the efficient harmonic generation is achieved by a novel structure.
As conventional technique for wavelength conversion by means of the nonlinear optical effect, gas lasers or solid state lasers have been experimentally employed in such a manner that the light therefrom is used as a fundamental wave to obtain harmonics. In actuality, apparatuses of this type have been put to practical use.
The apparatuses presently used are, however, poor in conversion efficiency, and large in size and thus not very portable. In addition, the apparatuses presently used are expensive and therefore used only in limited institutions, such as factories, universities, and laboratories.
Semiconductor lasers, on the other hand, have superior advantages, for example, small size, lightweight, high-efficiency, long service life, and low price, and experimental work has been pursued to obtain a harmonic using the light of a semiconductor laser as the fundamental wave. However, the light intensity of semiconductor lasers is not sufficiently strong and low in the harmonic generation efficiency and therefore has not been as yet put to practical use.
To obtain the prominent harmonic generation efficiency, it is necessary to maintain the phase matching between the fundamental wave and the generated harmonic. If the fundamental wave and the harmonic are not matched in phase, they are canceled out because the harmonic is generated at various points where the fundamental wave is propagated.
In conventional apparatuses for performing wavelength conversion by means of the nonlinear optical effect, several techniques have been employed to obtain phase matching between the fundamental wave and harmonic, such as (1) a technique utilizing birefringence of a nonlinear optical crystal, and (2) a technique utilizing the difference of dispersion between the fundamental wave and harmonic propagated through a waveguide according to propagation modes, caused depending on the waveguide structure (disclosed, e.g., in OYO BUTSURI, vol. 56, No. 12 (1987), pp. (49-53).
For the former technique, however, it is essential to use a material which meets the phase matching conditions (Type I and Type II phase matching conditions) determined by the nonlinear susceptibility and birefringent index of the crystal. Moreover, it is necessary to use a bulk single crystal of a nonlinear optical material, and strictly adjust the direction and polarization of incident light and the axis of the crystal. Further, precise temperature control is required to eliminate the dependence of the refractive index on temperature. This method is used to achieve the phase matching in an SHG unit for a high-power laser. However, this unit is large-sized and expensive, and, if used as an SHG unit for a semiconductor laser, the advantages of the semiconductor laser, i.e., small size, lightweight, low price, etc., are not available.
As for the latter technique utilizing the dispersion difference, it is very difficult to produce a waveguide having dimensions coincident with the theory, with satisfactory reproducibility.